Who exactly has the liability when the on site regulations don't apply to an sub contractor using diff rules?

now then been trying to get an answer for a while. Lets say that I use a regular contractor for delivery of certain items, and that this contractor has their own hse working practices for the delivery and installment of said items. Now I have my own onsite regulations regarding this, but the two practices are not the same. considering its written into the agreement that they will use their own policy on hse, who would be liable for an onsite accident which was incidently their own fault?